Pluto map information

On the left you see all of the maps I've generated
of Pluto over the years.

Mutual event map

The map at the top (labeled MaxEnt - V) was based on
photometry of Pluto taken between 1954 and 1990. This work was published in 1992. Photometry by itself
cannot easily reveal a map of the surface, instead it just tells you
how bright the visible hemisphere is. But, if you have data over a long
enough time, and the surface does not change, and you get mutual eclipses
and occultations between Pluto and its big satellite, Charon, you can
detangle the information to get a map. During this time Pluto ranged from
35 AU (1954) to 29.6 AU (1989 at perihelion) while the sub-solar latitude
changed from -55 degrees (south latitude) to an equatorial view (0 degree
latitude). Over this time we were watching as the north pole was coming
into view after a century of being in the dark.

FOC/HRC map

The next two panels show reprocessed data from HST that were taken
between 1994 June 20 and 1994 Jul 01 when Pluto was at 29.8 AU and a
latitude of 12.7 degrees. At this time our the northern pole really
begins to come into view while the southern pole begins to slip
into winter. Two maps are shown for this epoch, one in ultraviolet
light (F278M) and one in blue light (F410N). These data were first published in 1997 and a revised analysis
was published in 2010.

ACS/HRC map

The bottom two panels show the most recent data from the Hubble Space
Telescope (HST) where 394 images were combined to create an albedo
map of Pluto in blue (F435W) and green (F555W) light. The data were
taken between 2002 Jun 11 and 2003 Jun 08 when Pluto was 30.6 AU from
the sun. During this time we were viewing Pluto from a latitude of
roughly 30 degrees. That means within 30 degrees of the south pole we
can't see the surface and that part of Pluto gets no sunlight (just like
Antarctica in the winter). Between these two epochs the distance from
the Sun changed by 2.5% but the area of the unilluminated polar region
increased by a factor of four. These maps were
published in 2010.

Comparisons

One reason for extracting maps of Pluto's surface so many times is
just to try out new techniques and tools. The Hubble Space Telescope
wasn't available prior to 1991 and it clearly was a tool worth using
on this problem. Another reason for multiple maps is just to look to
see if anything is changing on the surface. We know there is a lot
of nitrogen ice along with methane ice that's mixed in. These ices
must move around on the surface with the progression of the seasons.
We've seen something like this on Mars with the carbon dioxide ice
moving back and forth between its poles during the Martian year. Well,
this process must also happen on Pluto. In the case of Pluto, its year
is much longer (248 years) but the seasonal cycle is much more extreme
thanks to Pluto's eccentric orbit and its large obliquity (that's the tilt
of the pole relative to the orbit, for Earth it's 23 degrees, for Pluto
it's nearly 80 degrees). The big question in my mind for the past three
decades that I've been working on this is does the seasonal cycle change
in the surface enough to see it from our remote vantage point on Earth?
There have been many theories published that tell us change is likely
but we've had very little data from which to say which theories (or even
guesses sometimes) apply to Pluto.

The best pair of maps to use in looking for change are from the FOC F410N
and ACS F435W data. These two maps are very similar in resolution
and wavelength of light used. It's not hard to see that there are large
scale changes that are evident here. The most striking change is at the
top (north) of the maps. Between 1994 and 2002 the north pole of Pluto
has brightened though other changes are evident as well. A brightening
of the north pole is a little odd. I imagine that a lot of clean surface
ice accumulated on the north pole during its winter season. Now it's coming
into view and getting warmed up. I would expect the surface to start out
bright and then gradually dim as ice sublimates away and moves to the other
pole. But, that's not what is happening and it will be a very important
clue to understanding seasons on Pluto and how the surface and its
atmosphere interact.

Color map

I've tried many times to estimate what Pluto would look like if
you were there in person. These new maps give me the best data yet to
do this. The two maps I have give me the reflectivity of every spot on
the surface but at only two wavelengths. To get Pluto's true color I
really need to know the reflectivity at all colors visible to the eye.
Thus to get true color I have to make some assumptions. To do this I
guess there are two sorts of things on the surface that give rise to its
color: neutral colored ice (can be dark ice or bright ice) that has no
color at all, and, red gunk. We know these two components are on the
surface and we have good guesses for their individual spectra. I then use
the maps to estimate how much of these two things are present at each point
on the surface. From that estimation I can then compute the color. The map
below shows this true color rendition.

There's one peculiar thing about this map and it has nothing to do with
Pluto. There are regions that look (to me) like they have a slightly
bluish cast. Well, those regions are actually gray, not blue at all. But,
when you put that "color" next to the other yellow-orange color it looks
blue by contrast. Strange.

The map above shows extremes in brightness (albedo). The darkest areas
are as dark as coal, reflecting only a few percent of the light that hits it.
The brighest areas are almost perfectly reflecting. There is just as much
contrast on Pluto as we've seen on the surface of Saturn's moon Iapetus. That
alone make Pluto stand out from most things in the solar system. Looking a
little closer we see that the north polar regions are generally more
neutral in color and brighter than average for Pluto. This is the area that
is recently emerged from a 120-year long winter. This color and albedo is
consistent with it being a region covered in frost deposits, presumed to be
nitrogen and methane ices. Moving toward the equator the trend is toward
more albedo variations and more color diversity and that trend continues
right up the the southern-most region we can see. We won't know for sure why
Pluto looks like this until the New Horizons spacecraft gets there in 2015
but it's going to be fun to try and work this out.